traces: use colors for the semantic values

This makes reading the trace slightly easier.  It would be very nice
to highlight the "big steps", especially reductions.  But this is a
private experiment: do not use it.

* data/diagnostics.css (value): New.
* src/parse-gram.y: Use no delimiters and no c quotation for strings
to facilitate debugging.
(tron, troff, TRACE): New.
Not very elegant, but until there is support for printf-formats in
libtextstyle, it shall be enough.

data/skeletons/c.m4

@@ -544,8 +544,10 @@ m4_if(b4_skeleton, ["yacc.c"],
     YYPRINT (yyo, yytoknum[yytype], *yyvaluep);
 # endif
 ]])dnl
-  b4_symbol_actions([printer])[
-}
+b4_percent_code_get([[pre-printer]])dnl
+  b4_symbol_actions([printer])
+b4_percent_code_get([[post-printer]])dnl
+[}
 
 
 /*---------------------------.

data/skeletons/lalr1.java

@@ -87,8 +87,8 @@ b4_output_begin([b4_parser_file_name])[
 ]])[
 ]b4_user_pre_prologue[
 ]b4_user_post_prologue[
-]b4_percent_code_get([[imports]])
-[/**
+]b4_percent_code_get([[imports]])[
+/**
  * A Bison parser, automatically generated from <tt>]m4_bpatsubst(b4_file_name, [^"\(.*\)"$], [\1])[</tt>.
  *
  * @@author LALR (1) parser skeleton written by Paolo Bonzini.